This present invention generally relates to a new class of liquid crystalline monomers and polymers. More in particular, it describes polymerizable dioxetane compounds which are capable of forming liquid-crystalline polymeric networks with advantageous properties which make them useful inter alia in optical display units (LCDs).
Oriented structures are of great interest due to their anisotropic properties. Such structures can possess highly anisotropic optical, electrical and thermo-electrical properties which are desirable for various applications.
Liquid-crystalline (LC) molecules combine liquid-like properties such as low viscosity, with crystal-like properties such as anisotropy. LC phases are observed between the melting temperature and the anisotropization temperature. There are various LC phases with different kinds of ordering, of which the nematic an cholesteric phases are of special interest. In the nematic phase, only a degree of orientational order is present and the molecules tend to orient their long axis along a common direction (xe2x80x9cthe directorxe2x80x9d). In the cholesteric phase, molecules are oriented in nematic layers and the nematic director is rotated around a helix. This phase is obtained by the inclusion of chiral molecules in a nematic system. Characteristic for this phase is the property of selective reflection of a band of circularly polarized light which makes the material looks colored. This property of the cholesteric phase can be used in the production of passive optical components.
In an LC phase, molecules are oriented in small domains. The orientation of these domains can be achieved in an electric field, a magnetic field or on specially treated surfaces. For example, in a sufficiently high electric field (1 V/xcexcm), molecules with a positive dielectric anisotropy can be oriented along the field. Surfaces coated with a polymeric layer which are subsequently rubbed, tend to orient LC molecules along the direction of rubbing. Surfaces treated with surfactants tend to orient LC molecules perpendicular to the substrate. These properties make LC molecules extremely versatile in obtaining any orientation on any surface geometry.
LC phases and the properties of LC systems within a certain phase are highly temperature dependent. Therefore, any attempt to xe2x80x9cfreeze inxe2x80x9d the properties of an LC system by polymerization has to be carried out isothermally. Changes in temperature may result in polymerization in the wrong phase or in a poorly oriented state. For these reasons isothermal photopolymerization, and also e-beam polymerization, of LC molecules are the preferred methods. For the purpose of photopolymerization, LC molecules with polymerizable groups, such as LC acrylates, epoxides and vinylethers are usually employed, together with an appropriate photo-initiator. Using monomers with two or more polymerizable groups will lead to the formation of crosslinked networks with improved thermal and chemical stability. The LC system is heated to the desired temperature and after the disappearance of the domains (induction of long range ordering) polymerization is initiated with a UV-light source or the like, to obtain a three-dimensional network in which the orientation of the molecules and the properties of the desired phase are xe2x80x9cfrozen inxe2x80x9d. For further background information, reference may be made e.g. to Hikmet R. A. M. et al., Prog. Pol. Sci. 21:1165-1209 (1996).
Recent prior art predominantly discloses LC mono- and diacrylates for use of photopolymerizable liquid crystals in the manufacture of optical components to be used in liquid crystal displays (LCDs). See, for example, WO 96/24647, WO 97/00600, WO 98/47979. Drawbacks of the use of this type of materials frequently include the following: (1) the polymerization reaction is inhibited by oxygen which demands the need of an inert gas during the polymerization process; (2) due to the relatively high crystallization temperatures of the liquid crystals the processing has to be done at high temperatures which makes film making by spincoating nearly impossible due to prior crystallization; (3) the polymer network prepared from acrylates will exhibit stress at room temperature due to polymerization shrinkage, which may also lead to (4) deformation of the optical component in which the polymer network is applied.
In the past decade, various authors disclosed liquid crystalline polyoxetanes based on mono-oxetane monomers, as well as their preparation and properties. See, for example, Kawakami Y., et al., Macromolecules 24:4531 (1991), Polym. Bull. 25:439 (1991), Polym. Int. 31:35 (1993), JP-A-06308462, JP-A-08020641, JP-A-08301859; Lu Y. -H., et al., Polymer Bulletin, 32:551-558 (1994), Macromolecules 28:1673-1680 (1995); Hsu L. -L., et al., J. Polym. Sci 35:2843-2855 (1997); Ogawa H., et al., Bull. Chem. Soc. Jpn. 70:1649-1657 (1997); JP-A-07225370.
The ring-opening polymerization reaction of mono-oxetanes, initiated by e.g. a BF3 etherate complex and resulting in side-chain liquid crystal polymers with flexible polymer backbones and broad liquid crystalline phase temperature ranges which extend from room temperature up to 240xc2x0 C., can be generally depicted as follows: 
wherein X generally represents a spacer group, such as an alkylene, an ether-based divalent group, or a siloxane-based divalent group, Y is a mesogenic group, and Z is a chiral group-containing terminal group.
However, the liquid crystalline polyoxetanes based on mono-oxetane monomers appear to suffer from some drawbacks, in that the stability of the alignment of the molecules is not so high due to thermal transitions or crystallization. Furthermore, the alignment of the polymers to obtain domain-free films is difficult due to the relatively high viscosity and the films are not stable to organic solvents.
There is therefore a need for photopolymerizable liquid crystalline compounds and polymerized derivatives thereof that do not have the disadvantages mentioned above, but rather exhibit improved properties, such as being polymerizable in the air, capable of being spincoated at room temperature, aligned easily, and showing very low polymerization shrinkage which would make them useful in a variety of applications.
It has now surprisingly been found that a new class of compounds, photo-polymerizable liquid crystal dioxetanes and/or polymeric products derived thereof show excellent properties which make them useful for application e.g. in liquid crystal mixtures and liquid crystal devices such as liquid crystal displays (LCDs).
Accordingly, the present invention provides in one aspect a compound having the general formula I: 
wherein R1 and R2 are independently selected from straight or branched C1-C4 alkyl and hydrogen;
X and Xxe2x80x2 are independently selected from oxygen, sulfur, a single covalent bond, xe2x80x94Oxe2x80x94COxe2x80x94, xe2x80x94COxe2x80x94Oxe2x80x94, and xe2x80x94Oxe2x80x94COxe2x80x94Oxe2x80x94,
Y and Yxe2x80x2 are spacer groups having each independently 1 to 30 carbon atoms (linear or branched), in which the carbon chain may be interrupted by oxygen in the ether function or by sulfur in the thioether function;
Z and Zxe2x80x2 are independently selected from oxygen, sulfur, a single covalent bond, xe2x80x94Oxe2x80x94COxe2x80x94, xe2x80x94COxe2x80x94Oxe2x80x94, and xe2x80x94Oxe2x80x94COxe2x80x94Oxe2x80x94, and
M is any suitable mesogenic group.
In another aspect, the invention provides liquid-crystalline compositions comprising one or more of said compounds of formula I, in conjunction with one or more liquid crystalline compounds selected from the group of normal non-polymerizable compounds and mono-oxetane monomers. In a preferred embodiment, said compositions comprise one or more chiral compounds.
In still another aspect, the invention provides a method of preparing the compounds of formula I as defined above, as well as a method of preparing said liquid-crystalline compositions comprising one or more compounds of formula I and, optionally, one or more chiral compounds.
In yet another aspect of the present invention the use is provided of one or more compounds of formula I, or compositions comprising one or more of said compounds of formula I, for the manufacturing, for example, of optical display units, optical precision units, sensors, cholesteric liquid-crystalline coloring agents, and pigments.